The present invention relates to the equipment for melting and casting metal in a vacuum, and more specifically to vacuum casting machines. It may be used to advantage in vacuum metallurgy employing electric furnaces where the operation of preparing the molds for casting and cooling of the castings is a rather time-consuming one. The most expedient way of using the invention is the incorporation of the machine disclosed into an electron beam melting plant producing castings predominantly in refractory and chemically-active metals, such as, for example, tungsten, tantalum, molybdenum, niobium, titanium, and their alloys.
There is known a vacuum casting machine comprising a metal-melting vessel disposed in a melting chamber fitted with means of heating the metal and a vacuum seal serving to close an opening through which the metal is being poured, a casting mold accommodated in a chamber of its own so arranged that an opening in this chamber in fact registers with the opening in the melting chamber, and a system for evacuating both the melting chamber and the chamber with the mold (USSR author's certificate No. 212,289).
The known machine serves the purpose of melting metal and is capable of heating the casting mold to approximately 1000.degree. C accompanied by the degassing of the material it is made of followed by the centrifugal casting of metal in the mold and cooling of the casting to between 200.degree. and 400.degree. C, all these operations being performed in a vacuum.
In the known machine the melting chamber is accommodated on supports and is a water-cooled vessel with heaters for the melting of metal in the form, for example, of consumable electrodes producing an arc. The vessel is connected to a vapor pour pump. One of the walls of the vessel is provided with a manhole used for servicing the items of equipment installed in the vessel. This manhole is tightly closed when a vacuum is applied to the vessel.
Rigidly attached at the bottom of melting chamber is a dome-shaped shell arranged so that its convex side faces the inside of chamber and provided with a centrally-located operating closed by a vacuum seal. The edge of shell extends beyond the chamber and forms a pipe which has a flange for connecting the chamber containing the mold. The space inside the shell is connected to a vacuum pumping system.
The metal-melting vessel accommodated in the melting chamber is a water-cooled crucible provided with means of pouring the metal into the mold. Said means incorporates a shaft attached to the crucible with one of its ends whereas the other end extends through a vacuum seal beyond the melting chamber and is linked up with a self-contained drive. Installed in the melting chamber is also a funnel which both forms and directs the spout of molten metal as the metal is being poured into the mold. During this operation the funnel is inserted into the opening in the dome-shaped shell.
The casting mold is disposed in a separate chamber fitted with a flange for connecting to the flange of dome-shaped shell so that the shell and said chamber form a space separated from both the atmosphere and the space of melting chamber.
The mold-containing chamber is installed on a carriage and is capable of travelling with respect to the melting chamber along rectilinear rails placed under the melting chamber. Since the mold somewhat protrudes above the flange of its chamber, the carriage is fitted with jacks serving to move the mold-containing chamber vertically so as to connect its flange to the flange of dome-shaped shell.
For employing the centrifugal casting process, the casting mold is held fast to a base plate provided at the bottom of mold containing chamber. The base plate is attached to the end face of a shaft extending beyond the chamber by means of a vacuum seal. Fitted to the outside end of the shaft is a pulley linked up with an electric motor through a V-belt drive. The motor is installed on the carriage. The chamber is also fitted with heaters serving to heat the mold.
The known vacuum casting machine is provided with a self-contained vacuum pumping system serving to evacuate both the melting chamber and mould-containing chamber.
The vacuum pumping system of the melting chamber is a two stage one consisting of a high-vacuum vapor pump coupled to the melting chamber through a vacuum seal and a rough-vacuum mechanical pump producing a backing vacuum for the vapor pump.
The vacuum pumping system of the mold chamber is a single-stage one consisting of a rough-vacuum mechanical pump connected to the space inside the dome-shaped shell through a line because the mold chamber can be evacuated of air only if its flange is coupled to the flange of dome-shaped shell. All joints of the casting machine are provided with seals.
By the term high-vacuum pump as employed herein is meant a pump able to produce a pressure not below 5.times.10.sup.-3 mm Hg whereas the term rough-vacuum pump denotes a pump ensuring a pressure of up to 1.times.10.sup.-2 mm Hg. Said pumps maintain the requisite vacuum in the spaces evacuated throughout the operation.
The machine operates on the following lines. On sealing and evacuating the space of melting chamber, a casting mold is placed into the carriage-mounted chamber and secured to the base plate. The carriage is moved along the rails under the melting chamber so as to place the flange of mold chamber coaxially with the flange of the dome-shaped shell of melting chamber. At the next stage, the mold chamber is lifted with the aid of jacks so as to enable said flanges to couple one to another. A sealing member interposed between the flanges is compressed, sealing off the space formed by the dome-shaped shell and mold chamber. The mould chamber vacuum system is set into operation, evacuating said space and after that heaters are turned on for heating the mold to a temperature of approximately 1000.degree. C.
During the heating of mold, gases are gradually driven off the material of mold and to prevent the contamination of the space inside the melting chamber by said gases the vacuum seal in the opening of dome-shaped shell is closed before setting the mold chamber vacuum system into operation. The mechanical vacuum pump of said vacuum system removes the gases as they are being liberated from the material of mold. At the same time, the melting chamber vacuum system is set into operation.
On heating the mold to the temperature specified for the process, the rate of mold degassing slows down considerably, an increase in the vacuum in the dome-shaped shell indicating this condition. At this instant, the vacuum seal in the opening of dome-shaped shell is opened whereas the mold chamber vacuum system is disconnected and the vacuum in both the melting chamber and mold chamber is maintained with the aid of the high-vacuum vapor pump of the melting chamber vacuum system. A further increase in the vacuum brings about further degassing of the material of mold and an interval of time is required until this degassing is completed. After that a voltage is applied to the consumable electrodes and the melting of the charge, placed in advance into the melting vessel of the melting chamber, is started. When sufficient melt of specified chemical composition is accumulated in the melting vessel, a funnel serving to protect the vacuum seal from splashes and direct the spout of melt into the casting mold is inserted into the opening in the dome-shaped shell. To produce castings by the centrifugal casting process, the drive of the base plate shaft is set into operation with the casting mold secured to said base plate. The electrodes are removed from the melting zone and deenergized, and the pouring arrangement is set into operation to pour the mold.
On completing the pouring, the funnel is returned into its original position and the casting in the mold is cooled to a temperature at which the gases of the atmosphere have no harmful effect on the material of the casting. The cooling takes place in a vacuum, i.e., the mold chamber remains connected to the melting chamber.
During the process of cooling, the vacuum in mold chamber is maintained with the aid of the vapor pump of melting chamber; alternatively, if the vacuum seal in the dome-shaped shell is closed, the vacuum is produced by the mechanical pump of the mold chamber vacuum system. If necessary, the casting can be cooled under a blanket of an inert gas pumped into the dome-shaped shell while the vacuum seal is being closed.
On finishing with the cooling of casting, the mold chamber is lowered onto the carriage with the aid of jacks and wheeled out from under the melting chamber along the rails. After that, the mold with the casting is removed from the base plate to which it is held down and withdrawn from the chamber. A new mold is placed into the chamber and the cycle is repeated.
It will be noted that the known vacuum casting machine operates in several stages which are: the placing of the mold on, and securing to, the base plate in the chamber accommodated on the carriage; the wheeling of the carriage with the chamber under the melting chamber and coupling the mold chamber to the melting chamber in a vacuum-tight fashion; the heating of the mould and the degassing of mold material with the rough-vacuum pump set into operation; the final degassing of mold material with the vacuum seal in the dome-shaped shell held open and the evacuation of both the melting chamber and mold chamber with the aid of the high-vacuum pump; the melting of charge and pouring of the mold; the cooling of the casting; the disconnection of the mold chamber from the melting chamber and the wheeling of the carriage from under the melting chamber for the purpose of removing the casting.
The machine may feature more than one carriage with mold chambers. In this case there is a practical possibility to overlap in time to some extent the operations of removing the mold with cooled-down casting and of coupling a new chamber with a mold to the melting chamber.
The known vacuum casting machine suffers from a number of drawbacks, one of them being the fact that the mold chamber is of a layout which enables the operations of mold heating and degassing of mold material and also that of cooling to take place only if the mold chamber is connected vacuum-tightly to the melting chamber (there is no other way of evacuating the mold chamber except through the dome-shaped shell of melting chamber). For the same reason, the casting is cooled also while the chambers are connected one to another in a vacuum-tight fashion or otherwise the gases of the atmosphere will have an adverse effect on the material of the casting.
The period elapsed in heating the mold and degassing its material and also that required for cooling the casting by far exceeds the melting time. While casting such metals as tungsten, tantalum, molybdenum and niobium, the mold should be heated to more than 1000.degree. C and this heating along with the degassing of mould material takes over one hour. The process of final degassing is considerably extended in time if electron beam guns requiring a vacuum of the order of 1.times.10.sup.-3 mm Hg in the melting chamber are used as the means of melting the metal.
The period elapsed in cooling such castings to a temperature between 200.degree. and 400.degree. C, which is regarded as harmless from the standpoint of illeffects of the gases of the atmosphere on the material of the casting, exceeds two hours and is selected depending on the size and shape of casting and also the material of mold. This is hardly comparable with some 30 to 40 minutes which are commonly required to obtain the requisite amount of melt in the melting vessel and so the operation of melting cannot be even partly overlapped in time by the operations of heating the mold and cooling the casting, said design features of the mold chamber being the cause of this disparity.
It stands thus to reason the production rate of the known machine is in fact decided by the duration of the period elapsed in time-consuming auxiliary operations of heating and degassing the mould and cooling the casting, and this is a disadvantage of the known machine.
Another disadvantage of the known machine is the fact that the mould chamber is of the non-split type, rendering the access to the base plate for securing the mould thereto difficult. Used commonly in such a chamber for holding down the mould is a contrivance clamping the mould due to the action of the centrifugal forces coming into play when the base plate is set spinning. Said contrivance fails to give reliable performance because it cannot prevent the mold from moving away from the centre of bed plate under the influence of radial forces coming into existence if the casting in the spinning mold becomes even slightly non-symmetrical.